Highly Sensitive Measurement of Horseradish Peroxidase Using Surface-Enhanced Raman Scattering of 2,3-Diaminophenazine

The development of various enzyme-linked immunosorbent assays (ELISAs) coupled with surface-enhanced Raman scattering (SERS) detection is a growing area in analytical chemistry due to their potentially high sensitivity. A SERS-based ELISA with horseradish peroxidase (HRP) as an enzymatic label, an o-phenylenediamine (oPD) substrate, and a 2,3-diaminophenazine (DAP) enzymatic product was one of the first examples of such a system. However, the full capabilities of this long-known approach have yet to be revealed. The current study addresses a previously unrecognized problem of SERS detection stage performance. Using silver nanoparticles and model mixtures of oPD and DAP, the effects of the pH, the concentration of the aggregating agent, and the particle surface chloride stabilizer were extensively evaluated. At the optimal mildly acidic pH of 3, a 0.93 to 1 M citrate buffer, and AgNPs stabilized with 20 mM chloride, a two orders of magnitude advantage in the limits of detection (LODs) for SERS compared to colorimetry was demonstrated for both DAP and HRP. The resulting LOD for HRP of 0.067 pmol/L (1.3 amol per assay) underscores that the developed approach is a highly sensitive technique. We suppose that this improved detection system could become a useful tool for the development of SERS-based ELISA protocols.

Surface enhanced Raman scattering active substrate based on hydrogel microspheres for pretreatment-free detection of glucose in biological samples

Determining glucose in biological samples is tedious and time-consuming due to sample pretreatment. The sample is usually pretreated to remove lipids, proteins, hemocytes and other sugars that interfere with glucose detection. A surface-enhanced Raman scattering (SERS) active substrate based on hydrogel microspheres has been developed to detect glucose in biological samples. Due to the specific catalytic action of glucose oxidase (GOX), the high selectivity of detection is guaranteed. The hydrogel substrate prepared by microfluidic droplets technology protects the silver nanoparticles from the surrounding environment and improves the stability and reproducibility of the assay. In addition, the hydrogel microspheres have size-adjustable pores that selectively allow small molecules to pass through. The pores block the entry of large molecules, such as impurities, enabling glucose detection through glucose oxidase etching without sample pretreatment. This hydrogel microsphere-SERS platform is highly sensitive and enables reproducible detection of different glucose concentrations in biological samples. The use of SERS to detect glucose provides clinicians with new diagnostic methods for diabetes and a new application opportunity for SERS-based molecular detection techniques.

Recent advances in the application of Raman spectroscopy for fish quality and safety analysis

Fish is one of the highly demanded aquatic products, and its quality and safety play a pivotal role in daily diet. However, the possible hazardous substance in perishable fish both in pre- and postharvest periods may decrease their values and pose a threat to public health. Laborious and expensive traditional methods drive the need of developing effective tools for detecting fish quality and safety properties in a rapid, nondestructive, and effective manner. Recent advances in Raman spectroscopy (RS) and surface-enhanced Raman scattering (SERS) have shown enormous potential in various aspects, which largely boost their applications in fish quality and safety evaluation. They have incomparable merits such as providing molecule fingerprint information and allowing for rapid, sensitive, and noninvasive detection with simple sample preparation. This review provides a comprehensive overview focusing on the applications of RS and SERS for fish quality assessment and safety inspection, highlighting the hazardous substance and illegal behavior both in preharvest (veterinary drug residues and environmental pollutants) and postharvest (freshness and illegal behavior) particularly. Moreover, challenges and prospects are also proposed to facilitate the vigorous development of RS and SERS. This review is aimed to emphasize potential opportunities for applying RS and SERS as promising techniques for routine food quality and safety detection. PRACTICAL APPLICATION: With these applications, it can be clearly indicated that RS and SERS are promising and powerful in fish quality and safety surveillance, thereby reducing the occurrence of commercial fraud and food safety issues. More efforts still should be concentrated on exploiting the high-performance Raman instruments, establishing a universal Raman database, developing reproducible SERS substrates and combing RS with other versatile spectral techniques to promote these technologies from laboratory to practice. It is hoped that this review should arouse more research interests in RS and SERS technologies for fish quality and safety surveillance, as well as provide more insights to make a breakthrough.

Preparation of fluorescent bimetallic silver/copper nanoparticles and their utility of dual-mode fluorimetric and colorimetric probe for Hg2

A dual-mode colorimetric and fluorimetric probe was successfully established based on silver/copper bimetallic nanoparticles (AgCu-BNPs). The AgCu-BNPs were confirmed as individually bimetallic nanoparticles with a mean size of 7.7 ± 0.2 nm, as characterized by high resolution transmission electron microscopy. Intriguingly, the AgCu-BNPs possess both surface plasmon resonances (SPR) and fluorescence emission. AgCu-BNPs emanate bright blue fluorescence with optical emission centered at 442 nm with high quantum yield of 30.3%, and AgCu-BNPs were attenuated or even quenched by Hg²⁺ via both static and dynamic quenching, coincidently accompanied by a visible color change, which endow AgCu-BNPs a unique utility as dual-mode colorimetric and fluorimetric probes. The detection limits as low as 89 nM and 9 nM were determined by dual-mode of AgCu-BNPs, respectively. The recovery rates in real samples were found to be 97.3-118.8%, and 89.5-112.7% by colorimetric and fluorescent methods separately, demonstrates the good environmental tolerance of the dual-mode probe.

Fluorescein Hydrazide-Appended Metal-Organic Framework as a Chromogenic and Fluorogenic Chemosensor for Mercury Ions

In this work, we prepared a fluorescein hydrazide-appended Ni(MOF) (Metal-Organic Framework) [Ni3(BTC)2(H2O)3]·(DMF)3(H2O)3 composite, FH@Ni(MOF). This composite was well-characterized by PXRD (powder X-ray diffraction), FT-IR (Fourier transform infrared spectroscopy), N2 adsorption isotherm, TGA (thermogravimetric analysis), XPS (X-ray photoelectron spectroscopy), and FESEM (field emission scanning electron microscopy). This composite was then tested with different heavy metals and was found to act as a highly selective and sensitive optical sensor for the Hg2+ ion. It was found that the aqueous emulsion of this composite produces a new peak in absorption at 583 nm, with a chromogenic change to a pink color visible to the naked eye upon binding with Hg2+ ions. In emission, it enhances fluorescence with a fluorogenic change to green fluorescence upon complexation with the Hg2+ ion. The binding constant was found to be 9.4 × 105 M-1, with a detection limit of 0.02 μM or 5 ppb. This sensor was also found to be reversible and could be used for seven consecutive cycles. It was also tested for Hg2+ ion detection in practical water samples from ground water, tap water, and drinking water.

Chemometrics coupled 4-Aminothiophenol labelled Ag-Au alloy SERS off-signal nanosensor for quantitative detection of mercury in black tea

Black tea like other food crops is prone to mercury ion (Hg²⁺) contamination right from cultivation to industrial processing. Due to the dangerous health effects posed even in trace contents, sensitive detection and quantification sensors are required. This study employed the surface-enhanced Raman scattering (SERS) enhancement property of 4-aminothiophenol (4-ATP) as a signal turn off approach functionalized on Ag-Au alloyed nanoparticle to firstly detect Hg²⁺ in standard solutions and spiked tea samples. Different chemometric algorithms were applied on the acquired SERS and inductively coupled plasma-mass spectrometry (ICP-MS) chemical reference data to select effective wavelengths and spectral variables in order to develop models to predict the Hg²⁺. Results indicated that Ag-Au/4-ATP SERS sensor combined with ant colony optimization partial least squares (ACO-PLS) exhibited the best correlation efficient and minimum errors for Hg²⁺ standard solutions (R_c = 0.984, R_p = 0.974, RMSEC = 0.157 μg/mL, RMSEP = 0.211 μg/mL) and spiked tea samples (R_c = 0.979, R_p = 0.963, RMSEC = 0.181 μg/g and RMSEP = 0.210 μg/g). The limit of detection of the proposed sensor was 4.12 × 10^-7 μg/mL for Hg²⁺ standard solutions and 2.83 × 10^-5 μg/g for Hg²⁺ spiked tea samples. High stability and reproducibility with relative standard deviation of 1.14% and 0.84% were detected. The potent strong relationship between the SERS sensor and the chemical reference method encourages the application of the developed chemometrics coupled SERS system for future monitoring and evaluation of Hg²⁺ in tea.

Fast Activation and Tracing of Caspase-3 Involved Cell Apoptosis by Combined Electrostimulation and Smart Signal-Amplified SERS Nanoprobes

Caspase-3 is considered as one of the key proteases that can spontaneously regulate the life activities of cells, and its activation (usually is a slow process) will execute the apoptosis process of cells. Rapid activation of caspase-3 on demand in living-cells is therefore highly desired toward precise cancer therapy but it is still a key challenge. Herein, we applied electrostimulus (ES) to achieve fast activation of caspase-3 and trigger cell apoptosis, and developed a smart magnetic-plasmonic assembly nanoprobes (A-nanoprobes) to real-time trace cellular caspase-3 activation at the single cell level. The designer core-satellite A-nanoprobe, working specific to the activated caspase-3 via a disassembly tactic, provides strong "hot spots" to improve the sensitivity and therefore enables SERS sensing of cellular caspase-3 upon activated by ES. Single-cell analysis revealed that the ES can rapidly activate the apoptosis pathway of caspase-3 on demand to make the DNA fragmentation and ultimately induce the cell apoptosis. Such method and nanoplatform were further used to monitor ES-triggered caspase-3 activation in cell apoptosis process of different cell types, revealing that more caspase-3 will be activated for cancerous cells than normal cells during the ES to induce cells apoptosis. This strategy and platform are promising for detecting cellular caspase-3 and other enzymes in the process of cancer diagnosis and treatments.

Microfluidic paper-based analytical device using gold nanoparticles modified with N,N′-bis(2-hydroxyethyl)dithiooxamide for detection of Hg(ii) in air, fish and water samples

A new method for visual detection of mercury by color change is developed that can detect Hg²⁺ by the naked eye or a digital camera.

A novel and ultrasensitive yellow to taupe brown colorimetric sensing and removal method for Hg(ii) based on the thermosensitive poly(N-isopropyl acrylamide) stabilized silver nanoparticles

AgNP/PNIPAm was developed as a dual-functional colorimetric probe and removal system for Hg²⁺.

Controllable Synthesis of SERS-Active Magnetic Metal-Organic Framework-Based Nanocatalysts and Their Application in Photoinduced Enhanced Catalytic Oxidation

Fabrication of multifunctional nanocatalysts with surface-enhanced Raman scattering (SERS) activity is of vital importance for monitoring catalytic courses in situ and studying the reaction mechanisms. Herein, SERS-active magnetic metal-organic framework (MOF)-based nanocatalysts were successfully prepared via a three-step method, including a solvothermal reaction, an Au seed-induced growth process, and a low-temperature cycling self-assembly technique. The as-synthesized magnetic MOF-based nanocatalysts not only exhibit outstanding peroxidase-like activity, but can also be applied as a SERS substrate. Owing to these features, they can be used for monitoring in situ catalytic oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂ via a SERS technique, and the concentration of H₂O₂ was determined. Owing to the intrinsic character of the Fe-based MOF material (MIL-100(Fe)), a novel photoinduced enhanced catalytic oxidation effect was demonstrated, in which the catalytic oxidation of TMB and o-phenylenediamine was accelerated. This study provides a versatile approach for the fabrication of functional MOF-based nanocomposites as a promising SERS substrate with a unique photoinduced enhanced peroxidase-like activity for potential applications in ultrasensitive monitoring, biomedical treatment, and environmental evaluation.

Novel ratiometric surface-enhanced raman spectroscopy aptasensor for sensitive and reproducible sensing of Hg2

It is important to precisely monitor mercury (II) ions (Hg²⁺) for environment protection and human health monitoring. Although many strategies have been developed in the past decades, there still remains a challenge for developing an ultrasensitive, simple and reliable approach to detect Hg²⁺. Herein, we report a ratiometric surface-enhanced Raman scattering (SERS) aptasensor by employing aptamer-modified Au@Ag core-shell nanoparticles (Au@Ag NPs) as highly functional sensing probes, allowing for ultrasensitive detection of Hg²⁺. In principle, the thiolated 5'-Cy3 labeled aptamer probe (Cy3-aptamer) is firstly immobilized on the SERS substrate surface and then hybridizes with the 5'-Rox labeled complementary DNA (cDNA) to form a rigid double-stranded DNA (dsDNA), in which the Cy3 and Rox Raman labels are used to produce the ratiometric Raman signals. In the presence of Hg²⁺, the aptamer DNA turns into the thymine (T)-Hg²⁺-T mediated hairpin structure, leading to the dissociation of dsDNA. As a result, the Rox labels are away from the Au@Ag NP SERS substrate while Cy3 labels are close to it. Therefore, the intensity of SERS signal from Cy3 labels increases while that from Rox labels decreases. The ratio between the Raman intensities of Cy3 labels and Rox labels is linear with Hg²⁺ concentrations in the range from 0.001 to 1.0nM, and the limit of detection is estimated to be 0.4pM. The proposed strategy provides a new rapid, simple and reliable approach for sensitive detection of Hg²⁺ and may create a universal methodology for developing analogous aptasensors for a wide range of other analytes determination.

One-step fabrication and characterization of a poly(vinyl alcohol)/silver hybrid nanofiber mat by electrospinning for multifunctional applications

Preparation and application of a poly(vinyl alcohol)/silver hybrid nanofiber mat by electrospinning.

A Ni-based MOF for selective detection and removal of Hg2+ in aqueous medium: a facile strategy

A Ni-based 3D MOF, [Ni(3-bpd)₂(NCS)₂]_n, where 3-bpd is 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene, has been explored for simultaneous detection and removal of Hg²⁺ in aqueous medium with excellent Hg uptake capacity.

Surface enhanced Raman scattering based reaction monitoring of in vitro decyclization of creatinine → creatine

Raman signatures of decyclization of creatinine to creatine appear after 120 min at pH 8, 60 min at pH 10 and 30 min at pH 12. Signature of reversibility at later stages of the reaction.

Construction of highly sensitive surface-enhanced Raman scattering (SERS) nanosensor aimed for the testing of glucose in urine

A facile, highly sensitive “turn on” SERS nanosensor for the detection of glucose in urine has been developed based on the aggregation of the 4-mercaptophenylboronic acid decorated silver nanoparticles in the specific bonding to glucose.

Recent progress in detection of mercury using surface enhanced Raman spectroscopy--A review

Concerns over exposure to mercury have motivated the exploration of cost-effective, rapid, and reliable method for monitoring Hg(2+) in the environment. Recently, surface-enhanced Raman scattering (SERS) has become a promising alternative method for Hg(2+) analysis. SERS is a spectroscopic technique which combines modern laser spectroscopy with the optical properties of nano-sized noble metal structures, resulting in substantially increased Raman signals. When Hg(2+) is in a close contact with metallic nanostructures, the SERS effect provides unique structural information together with ultrasensitive detection limits. This review introduces the principles and contemporary approaches of SERS-based Hg(2+) detection. In addition, the perspective and challenges are briefly discussed.